IIS conflicts for port 80 with some applications: 503 Server unavailable or showing Windows Authentication system login dialog

Just spent another half of day solving the same problem already solved before.

If you have these symptoms:
* Http error 503 Server Not Available
or
* System login dialog even as you have no Windows authentication enabled

First of all, look at MS SQL Server Reporting Services configuration.
It creates TWO sites during install at localhost:80/Reports and localhost:80/ReportServer.

You should move both of them to other ports using Reporting Services Configuration Manager utility

The deep root reason is described here:
https://blogs.msdn.microsoft.com/webtopics/2010/02/17/a-not-so-common-root-cause-for-503-service-unavailable/

Another reason possible is Skype messenger. It also likes to take the port 80 preventing others from using it.

Good luck!

PS: Don 't forget about another very simple reason for Error 503:
You might be running your site in IIS under an Application Pool with user account credentials, and your password has just expired!